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Geometry Difficulty 5.7 AIME, harder Find the answer

6.1. In an isosceles triangle ABCABC, one of the angles is equal to the difference of the other two, and one of the angles is twice another. The angle bisectors of angles AA, BB, and CC intersect the circumcircle of the triangle at points LL, OO, and MM respectively. Find the area of triangle LOMLOM, if the area of triangle ABCABC is 8. If the answer is not an integer, round it to the nearest integer.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Answer: 11. Solution. Let in triangle ABCABC the angle α\alpha be equal to the difference of angles βγ\beta-\gamma. Since α+β+γ=180\alpha+\beta+\gamma=180^{\circ}, then βγ+β+γ=180\beta-\gamma+\beta+\gamma=180^{\circ} and therefore β=90\beta=90^{\circ}. If β=90\beta=90^{\circ} is twice one of the angles α\alpha or γ\gamma, then the triangle will be isosceles, which contradicts the condition. Let for definiteness α>γ\alpha>\gamma. Then β=90,α=60,γ=30\beta=90^{\circ}, \alpha=60^{\circ}, \gamma=30^{\circ}.

By the inscribed angle theorem:

MLC=MBC=45,CLO=CAO=30\angle MLC = \angle MBC = 45^{\circ}, \angle CLO = \angle CAO = 30^{\circ}. Therefore, MLO=75\angle MLO = 75^{\circ}. Similarly,

LOM=LOA+AOM=LCA+ABM=15+45=60\angle LOM = \angle LOA + \angle AOM = \angle LCA + \angle ABM = 15^{\circ} + 45^{\circ} = 60^{\circ},

!
OML=OMB+BML=OAB+BCL=30+15=45\angle OML = \angle OMB + \angle BML = \angle OAB + \angle BCL = 30^{\circ} + 15^{\circ} = 45^{\circ}.

Thus, the angles of triangle LOMLOM are α1=75,β1=60,γ1=45\alpha_{1}=75^{\circ}, \beta_{1}=60^{\circ}, \gamma_{1}=45^{\circ}.

Using the sine rule, we get the formula for the area of a triangle inscribed in a circle of radius R:S=12absinγ=122Rsinα2Rsinβsinγ=2R2sinαsinβsinγR: S=\frac{1}{2} a b \sin \gamma=\frac{1}{2} \cdot 2 R \sin \alpha \cdot 2 R \sin \beta \cdot \sin \gamma=2 R^{2} \sin \alpha \sin \beta \sin \gamma.

Therefore, SLOMSABC=2Rsinα1sinβ1sinγ12Rsinαsinβsinγ=sin75sin60sin45sin90sin60sin30=2sin75\frac{S_{LOM}}{S_{ABC}}=\frac{2 R \cdot \sin \alpha_{1} \sin \beta_{1} \sin \gamma_{1}}{2 R \cdot \sin \alpha \sin \beta \sin \gamma}=\frac{\sin 75^{\circ} \sin 60^{\circ} \sin 45^{\circ}}{\sin 90^{\circ} \sin 60^{\circ} \sin 30^{\circ}}=\sqrt{2} \cdot \sin 75^{\circ}. Since sin75=sin(45+30)=6+24\sin 75^{\circ}=\sin \left(45^{\circ}+30^{\circ}\right)=\frac{\sqrt{6}+\sqrt{2}}{4}, we get: SLOMSABC=3+12\frac{S_{LOM}}{S_{ABC}}=\frac{\sqrt{3}+1}{2}.

Therefore, SLOM=43+411S_{LOM}=4 \sqrt{3}+4 \approx 11.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.